This invention relates to integrated circuit components and methods for forming such components and more particularly to integrated circuit capacitors and methods for forming such capacitors.
Integrated circuit technology is developing to the point where there is an increasing need for capacitors having a high value of capacitance per unit area in both MOS and bipolar integrated circuits. For example in MOS D-RAMS, it is projected that for 4 Mbit circuits, the storage capacitors would need to have a unit capacitance of about 6.0 fF/um.sup.2 and operate at about 3.5 V. On the other hand, bipolar circuits also need capacitors having a high capacitance for switched load memory cells in order to reduce cell area, as well as to maintain speed and soft error immunity.
Looking ahead, it is also clear that the trend in future integrated circuits is toward higher lateral packing density and three dimensional circuits. In the near future however, it appears that in three dimensional circuit applications, more of the passive components, i.e. capacitors and resistors, would be located above the silicon substrate level which would be used for active transistors to maintain their performance. Consequently, the passive components would be constructed by thin film deposition technology by necessity.
In order to form capacitors having a high capacitance value, it is necessary to employ a dielectric having a high dielectric constant. In the past, this requirement has been at odds with the use of thin film technology because as the thickness of the dielectric film is reduced below 500A, its dielectric constant begins to drop. For example, tantalum pentoxide (Ta.sub.2 O.sub.5) has a very good dielectric constant, on the order of 25, in thick film form. However, thickness of capacitor dielectric material in silicon integrated circuits is preferably in the range of 50 to 500A. At a thickness on the order of 50 to 100A, the dielectric constant of Ta.sub.2 O.sub.5 drops down to around 6.0.
This apparent drop in dielectric constant is thought to be due to the formation of a series capacitor with other extremely thin dielectric films which are present on the substrate prior to the deposited dielectric formation. This can readily occur for dielectrics reactively formed over supposedly clean silicon substrates which have a thin native oxide present on the substrate surface. Since the dielectric constant of SiO.sub.2 is substantially less than materials like Ta.sub.2 O.sub.5, very thin layers of underlying oxide can have adverse effects on the measured capacitance.
In addition to problems caused by a significant drop in the value of the dielectric constant, thinness of the capacitors creates the requirement that they operate at fields in excess of about 1.0 megavolt per centimeter (MV/cm). Leakage currents at such high field strengths become a significant consideration. Indeed, for a given technology, the needs for high capacitance values and low leakages (or high operating voltages) are mutually opposed so that this has become a matter of compromise and design.
Another problem associated with thin film capacitors is their lack of symmetry. Symmetrical capacitors are those in which the capacitance remains relatively constant regardless of the magnitude or polarity of the applied voltage. If symmetry is required, it has been necessary to make the dielectric thicker to increase the breakdown voltage and decrease the leakage of the material at the expense of the capacitance per unit area. This dilemma becomes more significant for those capacitors in which the thin film dielectric is formed directly on silicon.